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Steve Chenoweth Office Phone: (812) 877-8974

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Steve Chenoweth Office Phone: (812) 877-8974

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  1. Below – One response to the need for change – denial! Whether you agree with the cartoon George W Bush here, or with the majority view, it’s certainly true that change is uncomfortable, and we often seek ways to resist change even when it’s needed. From http://www.globalchangeblog.com/2009/11/why-dont-people-engage-climate-change-part-5-a-perfect-storm-of-climate-change-denial/. Software Maintenance and EvolutionCSSE 575: Session 6, Part 2Problems with Changing Software - 1 Steve Chenoweth Office Phone: (812) 877-8974 Cell: (937) 657-3885Email: chenowet@rose-hulman.eduz

  2. We saw that almost all software changes… • Software Changes and Entropy Results • Ripple-Effects of Software Changes • Changes on Changes • Hydra-Effect… • What are some specific problems that occur? • What strategies work for each of them? Reprise - Symbol for Creative Entropy, a company who build iPhone and iPod apps. From http://www.creativeentropy.com/

  3. Feathers’ Change Tactics • Ch 6 – 23 • For now, we’ll talk about Ch 6 and parts of Ch 7, 8, and 9 as examples. • Suggested goals (see assignment for details) – • Pick one of two tactics from Ch 6 in Feathers, which looks like it would relate to your project, and study it. • Use that technique in making changes to your code. • Then pick another couple from Ch 7-23, and do the same thing. • Write about applying these, in your journal. • I think they’re straightforward to study and learn about. • I could put any of these on Exam 2, which, after all, is take-home / open book.

  4. Ch 6 Ch 6 – Making fast changes  While also reducing the risk of introducing errors with these … • Sprout methods and classes • Wrap methods and classes Right - Perfect implementation of this Chapter? – a wrap with sprouts.

  5. Ch 6 Sprouting Methods - start Given this code:  And a new “feature” goal -- to verify that none of the new entries are already in transactionBundle …

  6. Ch 6 Sprouting methods – “what to avoid” • How would many people try to add this?

  7. Ch 6 Sprouting methods – a better idea • Here we create a new method, just to handle the new “feature.” • Only an additional call line to it is needed in the existing method.

  8. Ch 6 Wrapping methods - start • Existing method adds up daily timecards for an employee and then sends their payment information to a PayDispatcher. • Need to add a new “feature”: To update a file with the employee’s name so that it can be sent off to some reporting software. Existing code:

  9. Ch 6 Wrapping methods - solution Existing code is now “wrapped”:

  10. Ch 7 Ch 7 – Speeding-up changes • As systems evolve, it takes longer and longer to understand how to make the next change. • But, once you find where to make a change, it should not necessarily grow increasingly complex to decide how to make the change. • This Ch is about how to optimize this second part of making a change…

  11. Ch 7 The role of lag time • It slows you down if there’s a delay in seeing if your fix works or not. • Feathers’ goal – See if you get feedback that your change worked in less than 10 seconds! • Psychologically, under 2 seconds would be ideal! • Think learning vocabulary from flash cards… Right – Mandarin Chinese flash cards. From http://www.semanda.com/.

  12. Ch 7 How to organize code for builds • Goal – make a cluster of classes build more quickly… • How to minimize what has to be rebuilt every time you recompile… • Typical trick – Extract interfaces for the classes in your cluster that are used by classes outside the cluster: • Replace references to a class with references to its interface. • Section-off the cluster to a new package or library for testing.

  13. Ch 7 An example to speed-up • We need to make some changes to one of these classes, AddOpportunityFormHandler, and • We’d like to make the build go faster, too.

  14. Ch 7 But… • All the classes that AddOpportunityFormHandler depends on are concrete! • And both ConsultantSchedulerDB and also AddOpportunityXMLGenerator could depend on other classes that aren’t in the diagram. • We first consider how to avoid ConsultantSchedulerDB, which is a parameter required for the AddOpportunityFormHandler’s constructor.

  15. Ch 7 Replace the DB class with an interface • This also can make the build faster, because changes to that interface don’t require recompiling AddOpportunityFormHandler (we have a layer of indirection between these).

  16. Ch 7 We can do this again… • With the OpportunityItem that’s created by the ConsultantSchedulerDB:

  17. Ch 7 We could show this via packages • Break up the cluster to show recompilation gains:

  18. Ch 7 Advantages • We have a package, OpportunityProcessing, that has no dependencies on the database implementation (just on the interfaces to it). • This is an application of the Dependency Inversion Principle: • When your code depends on an interface, dependency is usually very minor. • Your code doesn’t have to change unless the interface changes.

  19. Ch 7 We also can reduce the reverse dependencies • So that we make the builds faster for code that depends on our class, AddOpportunityFormHandler, by creating interfaces also from them back to it:

  20. Ch 7 Feathers’ conclusion • When you introduce more interfaces and packages into your design to break dependencies, • The amount of time it takes to rebuild the entire system goes up slightly. There are more files to compile. But, • The average time for a make, a build based on what needs to be recompiled, can go down dramatically.

  21. Ch 8 Ch 8 – How to add a feature • Alternatives beyond sprouting and wrapping… • If you keep the old code, it doesn’t get better! • Need to get tests in place to speed development. • Feathers’ formula for TDD includes, as a last step – • Remove duplication: • The first try at new code often includes things borrowed from elsewhere. • Do we keep these, or combine them? Etc.

  22. Ch 8 Programming by difference • Say we’ve tested Java class MessageForwarder, which has method getFromAddress: private InternetAddressgetFromAddress(Message message) throws MessageException{ Address [] from = message.getFrom (); if (from != null && from.length > 0) return new InternetAddress (from [0].toString ()); return new InternetAddress(getDefaultFrom()); • This code strips-out the “from” of a received message so it can be used as the “from” in the forwarded message to list recipients. • Used in just one place in current code.

  23. Ch 8 New requirement… • Suppose we now need to support anonymous mailing lists. • We think we can use almost the same MessageForwarder class – so we subclass it, and use it for the new service: MessageForwarder forwarder = new AnonymousMessageForwarder(); With the only difference in the subclass being its getFromAddress(Message) class (vs. the last slide).

  24. Ch 8 And the new method is… • Almost the same… protected InternetAddressgetFromAddress(Message message) throws MessageException{ String anonymousAddress = “anon-” + listAddress; return new InternetAddress(anonymousAddress); • The getFromAddress(Message) in the original MessageForwarderwould also now be protected vs private.

  25. Ch 8 This subclassing is a quick change • Gets code running fast • So long as the change isn’t part of a big tree of alternatives, in which case it may multiply the size of a family of classes! • For that situation, Feathers suggests parameters • Read from a file, or • From a separate class of their own, that’s just there to return parameter-dependent values

  26. Ch 8 This kind of subclassing breaks the LSP • Each subclass overrides the original behavior of what’s now a parent class. • Alternatively, need a more general parent class, have the children differ where they need to.

  27. Ch 9 Ch 9 – Test harness problems* • Irritating parameter • Hidden dependency • Construction blob • Irritating global dependency • Horrible include dependencies • Onion parameter • Aliased parameter Let’s look at this one *Note that you don’t have to use test-first for these to be useful! They all are practical in promoting unit testing or earlier integration testing, as well.

  28. Ch 9 Irritating global dependency • We have a system that records building permits for a governmental agency. One Java class is this: • We’d like to test instances of this class… • What’s to stop us?

  29. The dependency rears its head… Irritating global dependency, cntd Ch 9 This is a singleton, a global value

  30. Irritating global dependency, cntd Ch 9 Getting rid of it, for testing We only use this during testing! For production, it remains a singleton.

  31. Irritating global dependency, cntd Ch 9 How it’s used in testing…

  32. Ch 9 Notice the emphasis on ease of testing • Feathers believes ease of unit testing is almost the same as the question of reuse – • Classes that are not easy to test, because of test-first problems like the list on slide 10, also are not easy to extend with children

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